Electromechanical filters and resonators usually comprise one or more transducers for converting oscillatory wave energy from electrical to mechanical form and vice versa in combination with a mechanical resonating element coupled to the transducer or transducers. One such filter is described in U.S. Pat. No. 3,064,213 issued to W. P. Mason on Nov. 13, 1962 and assigned to the assignee hereof, and another such filter is, for example, described in U.S. Pat. No. 3,264,585 issued to W. Poschenrieder on Aug. 2, 1966. The former of these two known electromechanical filters comprises a series of elongated bars connected to a central strip at or near their midpoints. The bars vibrate in the flexural mode about the central strip. It has now been found that this known filter is subject to unwanted longitudinal vibrations along the central strip affecting its response and stability. The latter of these two known electromechanical filters comprises an input and an output resonator vibrating in the longitudinal mode by means of electrostrictive transducer elements attached to both sides thereof. Similar unwanted modes were found affecting the stability and operation of such a known filter.
One attempt at solving the foregoing problem is described in U.S. Pat. No. 3,638,145 issued to E. P. Papadakis on Jan. 25, 1972 and assigned to the assignee hereof. In such a known arrangement, unwanted longitudinal resonances are eliminated by using a reinforcing member along the central strip of the electromechanical filter. Although such a method is acceptable for certain applications, it is primarily concerned with unwanted longitudinal bending modes interfering with the efficient operation of the filter. Moreover, such a known solution cannot be used when a light and compact filter or resonator free of unwanted flexural modes is needed.